Diesel engines have higher efficiency than gasoline engines due to the increased compression ratio of the diesel combustion process and the higher energy density of diesel fuel. As a result, a diesel engine provides improved gas mileage than an equivalently sized gasoline engine.
The diesel combustion cycle produces particulates that are typically filtered from the exhaust gases. A diesel particulate filter (DPF) is usually disposed along the exhaust stream to filter the diesel particulates from the exhaust. Over time, however, the DPF becomes full and must be regenerated to remove the trapped diesel particulates. During regeneration, the diesel particulates are burned within the DPF to enable the DPF to continue its filtering function.
One traditional regeneration method injects diesel fuel into the cylinder after the main combustion event. The post-combustion injected fuel is expelled from the engine with the exhaust gases and is combusted over catalysts placed in the exhaust stream. The heat released during the fuel combustion on the catalysts increases the exhaust temperature, which burns the trapped soot particles in the DPF. This approach utilizes the common rail fuel injection system and does not require additional fuel injection hardware.
Engine emissions are typically reduced by lowering cylinder temperatures, which occur when ignition timing is retarded. Retarding ignition timing, however, triggers the combustion process at a non-optimal point. As a result, engine efficiency, fuel economy and/or performance are reduced.
Exhaust gas recirculation (EGR) is another, more preferable method for reducing engine emissions. EGR involves re-circulating exhaust gases back into the cylinders, which limits the amount of oxygen available for combustion and lowers cylinder temperatures. EGR enables ignition timing to remain at an optimum point, which improves fuel economy and/or performance.
Traditionally, the EGR system must be disabled during DPF regeneration to prevent the post-injected fuel from being re-circulated into the engine. The re-circulation of the post injection fuel may damage the engine and/or the EGR system. Because the EGR system is disabled during DPF regeneration, the engine emission rates may increase during DPF regeneration.